Sealed lead-acid cells (often termed "VRLA" cells, viz., valve-regulated lead-acid) are widely used in commerce today. As is known, sealed lead-acid cells utilize highly absorbent separators, and the necessary electrolyte is absorbed in the separators and plates. Accordingly, such cells may be used in any orientation without electrolyte spillage as would occur with a flooded electrolyte lead-acid battery. Such cells are normally sealed from the atmosphere by a valve designed to regulate the internal pressure within the cell so as to provide what is termed an effective "oxygen recombination cycle" (hence the use of the terms "sealed" and "valve-regulated").
The advantages that are provided by sealed lead-acid cells in comparison to conventional, flooded lead-acid batteries are substantial and varied. Sealed lead-acid technology thus offers substantial benefits by eliminating maintenance (e.g., cell watering), expense (e.g., acid additions and purchases), environmental (e.g., expensive waste treatment systems and air-borne acid mist) and safety (e.g., acid burns) concerns.
It is thus not surprising that sealed lead-acid cells are widely used today for various applications that have widely differing requirements. In one type of application, generally termed as stationary applications, lead-acid cells are used, for example, for load leveling, emergency lighting in commercial buildings, for standby power for cable television systems, and in uninterruptible power supplies. The uninterruptible power supply may be used to back up electronic equipment, such as, for example, telecommunication and computer systems, and even as a backup energy source for entire manufacturing plants. When the principal power supply to the electronic equipment has been cut off, such as during a power outage or other interruption, the sealed cells (typically many electrically connected together) provide a source of reserve power to allow the telecommunication or computer system to remain operational until the principal power supply can be restored. The uninterruptible power supply also will accommodate short, or intermittent, losses in power, so that the function of the electronic equipment will not be impaired during a brief power outage.
In addition, there are many applications where sealed lead-acid cells are used in what are termed as motive power application. Sealed lead-acid cells are thus used as the power source for electric vehicles, fork-lift trucks, and the like.
The performance requirements for these two basic types of applications vary significantly. On the one hand, stationary applications are generally float applications, i.e., the cells are generally on float (i.e., an external voltage supply connected to the cells is held slightly above the cell potential to maintain charge), with an occasional need for a deep discharge when the main power source fails or is otherwise interrupted.
On the other hand, motive power applications require repetitive deep discharges, down to a 80% depth of discharge or even somewhat greater. Suitable cells must thus be capable of enduring repetitive charge-deep discharge-charge cycling regimes for up to 500 cycles or even more. Indeed, it would be desirable to provide cells capable of enduring from 1,000 to 2,000 cycles.
The widely varying requirements for these many applications has presented substantial problems to manufacturers of sealed lead-acid cells and batteries.
This has been further complicated in that, for motive-power applications, the compartment for the motive-power source has most often been designed for the size of batteries using conventional flooded lead-acid batteries.
All of these concerns, and additional concerns, have presented an extremely challenging environment for sealed-lead acid cell and battery manufacturers. This environment has resulted in, to a large extent, custom designs which satisfy particular applications.
Generally, the grids used have been made by gravity casting techniques. It has, however, long been recognized that gravity casting techniques, which are semi-continuous at best, can cause several production problems. In the first place, gravity casting techniques are subject to various problems which result in scrap as well as lack of product consistency and the like. These problems include operator error; wide variation in grid wire thickness and hence overall weight due to mold coating variations and irregularities; substantial material handling in production and difficulty in automating such processes and the accompanying inconsistencies due to human error and the like.
A further complicating factor is the need to provide grids of various sizes so that the capacity and other electrical performance requirements for an individual cell for a particular application can be satisfied. One approach utilized has been to provide a series of grids having essentially constant width while varying the height of an individual grid and the number of plates used in a particular cell to achieve a variety of capacity and other electrical performance requirements. Such grids have been made by utilizing gravity casting and a number of molds.
Potentially, the use of any continuous process like continuous grid casting or other continuous expanded metal fabrication techniques to make battery grids should be capable of minimizing, if not eliminating, one or more of the problems associated with gravity casting techniques. Some of these same considerations are of concern in making lead-acid grids for flooded conventional batteries such as automotive batteries. There has been accordingly substantial interest and effort directed toward the use of such techniques over the years insofar as making grids for automotive-type applications.
Various continuous processes for making wrought grids are known. All of such processes include slitting and expanding steps and often include cold rolling a continuous strip to the thickness desired before such expanding and slitting steps are carried out. It is, however, often difficult to achieve grids having satisfactory microstructures, particularly for positive grids when cold rolling is used.
To avoid such difficulties, one particularly desirable approach utilizes a directly cast strip, i.e., a continuous strip that is directly cast from molten lead alloy into the thickness desired for making the grids. The casting process thus does not include any cold rolling or other reduction in the thickness of the strip from the cast thickness to the thickness desired for making the grid. Equipment for making a suitable directly cast alloy continuous strip for molten lead alloy is commercially available (Cominco Ltd., Toronto, Canada). In this regard, U.S. Pat. No. 4,315,357 to Laurie et al. illustrates, in general, the method and apparatus for making the expanded mesh strip necessary for making a continuously cast grid. Still further, U.S. Pat. No. 3,858,642 to Battiston et al. thus discloses an apparatus for delivering an alloy to a rotating continuous casting drum. U.S. Pat. No. 5,462,109 to Vincze et al. discloses a further method and apparatus for producing metal strips which can be expanded and shaped to form expanded mesh grids for use in plates for lead-acid batteries.
However, the use of such methods and apparatus for producing directly cast strips (i.e., as disclosed in the Vincze et al. '109 patent) has been principally restricted to positive and negative plates for lead-acid batteries which are significantly thinner than the plate thicknesses required for industrial battery stationary and motive power applications. More particularly, the use of directly cast strips has principally concerned making starting, lighting and ignition grids which typically have a thickness of less than about 0.040 inch. In contrast, for industrial battery applications, it is desired to utilize grids having a thickness of at least about 0.060 or 0.080 inch or so. Indeed, long-life, stationary power applications often require grid thicknesses of at least 0.120 inch and even greater. Typically required grid thicknesses for industrial applications thus range from about 0.060 to 0.120 inch for positive grids and from about 0.060 to 0.100 inch for negative grids.
Yet, despite the well known shortcomings of gravity casting and the knowledge of continuous processes for making grids for automotive applications, it is not believed that a suitable continuous process has been developed for making grids and plates for industrial cell/battery applications. There accordingly is a need which exists for grids for industrial lead-acid cells and batteries which can be made in a continuous fashion.
It is accordingly a principal object of the present invention to provide a commercially viable process for making grids suitable for lead-acid cells for industrial cell/battery applications using continuous grid manufacturing methods.
A further object provides a continuous process for making grids which can achieve grids of varying sizes so as to accommodate the electrical performance requirements of a wide variety of applications.
Other objects and advantages of the present invention will become apparent as the following description proceeds. While the present invention will be described herein principally in connection with making grids and plates for VRLA sealed lead-acid cells and batteries, it should be appreciated that this invention is equally applicable to making grids and plates for flooded electrolyte cells and batteries designed for use in industrial battery applications. Such applications are known, and some have been discussed herein. Indeed, the present invention is useful for making thick grids and plates for any desired lead-acid cell/battery application. Even further, the present invention may be used to make thick metal strips for any application.